A piston for a medicine syringe or medical syringe is required to have no interaction with a liquid medicine to be filled in a syringe barrel and further, to be equipped with the mutually contradictory properties (performances) of sealing property and sliding property within the syringe barrel. For pistons to be used in pre-filled syringes (in other words, container syringes) which contain liquid medicines filled beforehand therein and are finding increasing utility in recent years, these properties are required at still higher levels than those required for conventional syringe pistons. Pistons for such pre-filled syringes are, therefore, required to keep quality unchanged, to permit safe use over long term, to assure sealing property (safety) even for liquid medicines of high penetrating property, and moreover, to possess a similar level of sliding property as in conventional syringes.
With a view to meeting such requirements, some approaches have been proposed to date, including: externally fitting one or more ring members such as O-rings on a main body of a plastic-made piston to form a corresponding number of sliding surfaces (at which the piston is to be brought in contact with the inner wall of a syringe barrel) (JP-A-07-213611); providing plural annular seal portions in the form of ribs on a sliding surface to form annular grooves between the adjacent annular seal portions (JP-A-07-124257); and limiting an area of contact of a piston with a syringe barrel and the compression factor of the piston to specific ranges, respectively (JP-A-57-022766). These proposed approaches are, however, all insufficient to satisfy both sealing property and sliding property for liquid medicines of high penetrating property.
In the meantime, the present inventors disclosed, as a piston capable of achieving both high sealing property and sliding property for liquid medicines of such high penetrating property, a syringe piston that features at least one annular microgroove formed on a sliding surface of a liquid-contacting, leading end portion of the piston (JP-A-2003-190285). With this piston, however, lamination of a film on the surface of the piston results in the formation of wrinkles in the film on the surface of the piston due to a difference in shrinkage factor between a rubber material as a material of the piston main body and the material of the film laminated on the surface of the piston when the piston shrinks beyond a certain level. In some instances, the liquid medicine may therefore leak out through crevasses formed by and along the wrinkles. There is, accordingly, an outstanding desire for further improvements.